Peter Hain has promised to step up Labour's fight against the Liberal Democrats.

The Cabinet minister said on Monday that the third party is now a serious threat to the government that could let the Conservatives into power "though the back door".

The leader of the Commons promised to take the fight to Charles Kennedy's party, which he claimed is drifting to the right while promoting itself as a left wing alternative to Labour.

"The Lib Dems are the most aggressive, effective, street politics campaigners and they have to be taken on," he said in an interview with the Independent.

"Progressive voters and Labour supporters have absolutely no idea that Lib Dems are no longer the really progressive alternative to Labour.

"They have positioned themselves to the left of Labour on some issues - like Iraq.

"But on other issues their right-wing lurch is putting them in a crypto-Thatcherite camp which would horrify any Labour supporter tempted to vote for them to send us a message."

Marginals

Hain argued that Labour can no longer afford to go soft on the Lib Dems as it has done in the past out of a belief that they are more dangerous to Tory MPs.

He claimed that the smaller margins between the parties mean that small swings away from the government could deliver the Conservatives power by default.

"I can foresee a situation where even a hob-nailed Tory party led by Michael Howard could do much, much better than anyone imagines," Hain said.

"In a whole string of marginal seats, where the Tories have been dismissed as not a serious threat to us, you could wake up the next morning with a Tory MP with even a small swing from Labour to the Lib Dems."

"The Lib Dems can't win the Tory-Labour marginals that exist by the score," he added.

"But Labour supporters, by plumping for [the Liberal Democrats] or staying at home, could inadvertently wake up with a Tory MP and therefore potentially let Michael Howard in through the back door."

'Blue murder'

Himself a former Young Liberal, Hain pledged to start "exposing" the Lib Dems at their annual conference later this month.

"There has got to be a really serious wake-up call to progressive voters and natural Labour supporters," he said.

"If they don't want a Tory government, there is really only one way to achieve that in Labour-held seats - and that is to vote Labour.

"It's important that the searchlight is now put on Lib Dem policies, starting at their conference. They have got away with blue murder for decades."

"There is a serious job to be done in exposing where the Liberal Democrats are now heading," he added.

"We can't just pretend the fight is against the Tories. They are the main enemy, but there is a second enemy close behind yapping at their heels."

Response

Responding to the onslaught, Lib Dem parliamentary party chairman Matthew Taylor said: "Peter Hain has given the game away - Labour are running scared of the Liberal Democrats, not least in Hartlepool. 

"It is Blair who has lost the trust of traditional Labour voters by moving to the right, by offering false choices in health and education, and by taking Britain to war in Iraq.

"In Labour's home territory, as well as the Tory heartlands, the Liberal Democrats are the party making gains, and the other two parties know it."